Vehicle state estimation apparatus and method

ABSTRACT

The present disclosure relates to an apparatus (1) for estimation of a vehicle state. The apparatus (1) includes a controller (21) configured to determine a first estimation of the vehicle state in dependence on at least one first vehicle dynamics parameter. A filter coefficient (FC) is calculated based on a first vehicle operating parameter. An operating frequency of a first signal filter (35) is set in dependence on the determined filter coefficient (FC) and the first estimation is filtered to generate a first filtered estimation of the vehicle state. The present disclosure also relates to a vehicle; and to a method of estimating a vehicle state.

TECHNICAL FIELD

The present disclosure relates to apparatus for estimation of a vehicle state; to a vehicle incorporating vehicle state estimation apparatus; to a dynamic filtering apparatus; to a method of estimating a vehicle state; and to a method of performing dynamic filtering.

BACKGROUND

The instantaneous state of a vehicle is defined by state parameters for vehicle pitch, vehicle roll and vehicle yaw. The vehicle state changes continuously while the vehicle is in motion, for example due to acceleration/deceleration of the vehicle and changes in the gradient of the surface on which the vehicle is travelling. The vehicle state is used by on-board vehicle dynamic control systems, for example to control vehicle stability.

In the automotive field it is known to employ an inertial monitoring unit (IMU) to continuously monitor vehicle acceleration in six degrees of freedom to monitor the vehicle state. It would be desirable to obtain more reliable data from the IMU and potentially to simplify the IMU by reducing the number of degrees of freedom in which acceleration and rates are measured. One approach is to estimate the vehicle state using measured dynamic parameters. However, global state estimations calculated from vehicle sensors are susceptible to noise and, in certain conditions, high error levels. Signal filtering can be used to lessen undershoots and overshoots in these conditions. However, for large signal-to-noise ratios heavy filtering is required which result in poor estimations in transient conditions.

It is against this backdrop that the present invention(s) has been conceived. At least in certain embodiments, the present invention seeks to overcome or ameliorate some of the shortcomings associated with known vehicle state estimation systems.

SUMMARY OF THE INVENTION

Aspects of the present invention relate to apparatus for estimation of a vehicle state; to a vehicle incorporating vehicle state estimation apparatus; to a dynamic filtering apparatus; to a method of estimating a vehicle state; and to a method of performing dynamic filtering.

According to a further aspect of the present invention there is provided apparatus for estimation of a vehicle state, the apparatus comprising a controller configured to:

-   -   determine a first estimation of the vehicle state in dependence         on at least one first vehicle dynamics parameter;     -   determine a filter coefficient in dependence on a first vehicle         operating parameter;     -   set an operating frequency of a first signal filter in         dependence on the determined filter coefficient and use the         first signal filter to filter the first estimation to generate a         first filtered estimation of the vehicle state; and     -   output a control signal in dependence on the first filtered         estimation of the vehicle state. In use, the operating frequency         of the first signal filter can be adjusted dynamically. By         dynamically changing the operating frequency, an improved         vehicle state estimation can be provided at least in certain         embodiments. By identifying when large signal-to-noise ratios         will occur, a strategy can be implemented based on the first         vehicle operating parameter which utilizes a continuously         varying filter. At least in certain embodiments, the signal         filter can be controlled to capture transient vehicle states         while removing erroneous information. The first filtered         estimation can be output as a first filtered estimation signal         for use by a vehicle stability control system.

The at least one first vehicle dynamics parameter can each be measured, for example by one or more vehicle sensors. The controller can be configured to receive a vehicle dynamics signal from each sensor. The at least one first vehicle dynamics parameter can be one or more of the following set: reference velocity, longitudinal velocity, longitudinal acceleration, lateral velocity, lateral acceleration, vertical velocity, vertical acceleration, roll, yaw, pitch and wheel slip.

The first estimation of the vehicle state can be generated using an appropriate vehicle state estimation algorithm. The vehicle state could be a roll angle of the vehicle measured about a longitudinal axis of the vehicle, which could be estimated using the lateral acceleration and/or the lateral velocity of the vehicle. Alternatively, the vehicle state can be a pitch angle of the vehicle and the at least one first vehicle dynamics parameter can include a reference velocity along a longitudinal axis of the vehicle. The pitch angle can be a global pitch angle of the vehicle. The global pitch angle is the angle of a longitudinal axis of the vehicle body relative to a horizontal reference plane. The global pitch angle can be calculated using the following global pitch estimation algorithm:

$\theta_{y} = {\sin^{- 1}\left( \frac{a_{x} - \overset{.}{u} + {\omega_{z} \cdot v_{y}}}{g} \right)}$

Where: θ_(y) is the global pitch angle;

-   -   a_(x) is the measured longitudinal acceleration;     -   {dot over (u)} is the derivative of the reference velocity U;     -   ω_(z) is the angular velocity about the Z axis;     -   v_(y) is the estimated lateral velocity; and     -   g is the acceleration due to gravity.

The first signal filter can be a low-pass signal filter and the operating frequency can be a cut-off frequency of the low-pass signal filter. The low-pass signal filter passes signals having a frequency lower than the cut-off frequency.

The controller can be configured to determine a second estimation of the vehicle state in dependence on at least one second vehicle dynamics parameter. An operating frequency of a second signal filter can be set in dependence on the determined filter coefficient. The controller can be configured to use the second signal filter to filter the second estimation to generate a second filtered estimation of the vehicle state.

The second estimation can be a relative body pitch angle of the vehicle. The relative body pitch angle is the pitch angle of the vehicle body in relation to a reference road axis determined by the gradient of the road on which the vehicle is travelling. The relative body pitch angle changes due to dynamic loads (such as acceleration/deceleration forces) compared to a static condition. The relative body pitch angle may also change to vehicle loads. The second estimation could be calculated based on the at least one second vehicle dynamics parameter. Alternatively, the second estimation can be determined by referencing the at least one second vehicle dynamics parameter to a look-up table stored in system memory. The at least one second vehicle dynamics parameter can include a longitudinal acceleration of the vehicle.

The second signal filter can be a high-pass signal filter, and the operating frequency can be a cut-off frequency of the high-pass signal filter. The high-pass signal filter passes signals having a frequency higher than the cut-off frequency.

The cut-off frequency of the low-pass signal filter can be set the same as the cut-off frequency of the high-pass signal filter. The low-pass signal filter and the high-pass filter thereby provide complementary filtering of said first and second estimations. By way of example, the cut-off frequency can be set in the range 0 to 1 Hertz; or in the range 0 to 0.7 Hertz.

The controller can be configured to combine the first and second filtered estimations to generate an output signal. For example, the controller can sum the first and second filtered estimations.

The first vehicle operating parameter can comprise a vehicle dynamics parameter or a vehicle control input. The first vehicle operating parameter could be a vehicle dynamics parameter which is either the same as or different from the at least one first vehicle dynamics parameter. Alternatively, the first vehicle operating parameter can be a vehicle control input, such as a throttle pedal position or a brake pressure.

The controller can generate a confidence value of the first estimation in dependence on the first vehicle operating parameter. The first confidence value can be generated in dependence on an absolute value of the first vehicle operating parameter. The first confidence value can be proportional (either directly or inversely) to the first vehicle operating parameter. The first confidence value can be generated in dependence on the rate of change of the first vehicle operating parameter. The first confidence value can be proportional (either directly or inversely) to the rate of change of the first vehicle operating parameter. A filter can be applied to the determined rate of change of the first vehicle operating parameter. The filter coefficient can be calculated based on said confidence value. The confidence value provides an indication of the confidence that the first estimation of the vehicle state is accurate.

The first vehicle operating parameter can comprise longitudinal vehicle acceleration. The controller can be configured to determine a rate of change of the longitudinal vehicle acceleration to generate the first confidence value of the first estimation. A high frequency filter can be applied to the determined rate of change of the longitudinal vehicle acceleration.

The first vehicle operating parameter can comprise a throttle pedal position. The controller can be configured to determine a rate of change of the throttle pedal position to generate a second confidence value of the first estimation. The controller can be configured to apply a high frequency filter to the determined rate of change of the throttle pedal position. The throttle pedal position generates a torque request signal for controlling operation of an internal combustion engine and/or an electric traction machine. In certain embodiments, the torque request signal would be equivalent to the throttle pedal position.

The first vehicle operating parameter can comprise brake pressure. The controller can be configured to analyse the brake pressure to generate a third confidence value of the first estimation. In certain embodiments, the position of the brake pedal would be equivalent to the brake pressure.

The first vehicle operating parameter can comprise at least one wheel slip measurement; and the controller can be configured to analyse the at least one wheel slip measurement to generate a fourth confidence value of the first estimation. The wheel slip measurement can be determined between wheels on the same side of the vehicle. Alternatively, the wheel slip measurement can be determined for laterally opposed wheels or diametrically opposed wheels. The at least one wheel slip measurement can be compared to a look-up table to generate the fourth confidence value. The analysis of the wheel slip measurement can comprise comparing first and second wheel slip measurements to a look-up table.

The controller can generate one or more confidence values based on different vehicle dynamic parameters and/or vehicle control inputs. The controller can be configured to normalize the one or more confidence values. The one or more confidence values can be normalized to fall within a predetermined range, for example a range of zero (0) to one (1) inclusive. A linear or non-linear gain can be applied to normalize each confidence value.

The controller can be configured to determine the filter coefficient in dependence on the generated confidence value. The controller can be configured to generate a plurality of said confidence values. The confidence values can each be generated in dependence on a different first operating parameter. The controller can be configured to generate the filter coefficient in dependence on the generated confidence value indicating the lowest confidence in the accuracy of the first estimation.

The controller can be configured to invert the generated confidence value. For example, the controller can subtract each confidence value from one (1). The filter coefficient can be generated in dependence on the inverted confidence value.

According to a further aspect of the present invention there is provided a dynamic filtering apparatus comprising a controller configured to:

-   -   generate a first signal and a second signal;     -   calculate a cut-off frequency;     -   apply the calculated cut-off frequency to a low-pass signal         filter and filter the first signal using the low-pass signal         filter;     -   apply the calculated cut-off frequency to a high-pass signal         filter and filter the second signal using the high-pass signal         filter; and     -   combine the filtered outputs of said low-pass signal filter and         said high-pass signal filter. The low-pass signal filter and the         high-pass filter thereby provide complementary filtering of the         first and second signals. The controller can be configured to         output a control signal in dependence on the combined filtered         outputs of said low-pass signal filter and said high-pass signal         filter.

The controller can be configured to generate the first signal in dependence on at least one first parameter. The at least one first parameter can be at least one operating parameter of a vehicle. The at least one operating parameter can be at least one first vehicle dynamics parameter, such as one or more of the following set: reference velocity, longitudinal velocity, longitudinal acceleration, lateral velocity, lateral acceleration, vertical velocity, vertical acceleration, roll, yaw, pitch and wheel slip.

The controller can be configured to generate the second signal in dependence on at least one second parameter. The at least one second parameter can be at least one operating parameter of a vehicle. The at least one operating parameter of the vehicle can be at least one second vehicle dynamics parameter, such as one or more of the following set: reference velocity, longitudinal velocity, longitudinal acceleration, lateral velocity, lateral acceleration, vertical velocity, vertical acceleration, roll, yaw, pitch, and wheel slip. The first and second vehicle dynamics parameters can be the same as each other or different from each other.

The controller can be configured to calculate a first confidence value of the first signal. The cut-off frequency can be calculated in dependence on said first confidence value. The controller can be configured to calculate the first confidence value in dependence on a third parameter. The third parameter can be an operating parameter of a vehicle. The operating parameter could be one of said vehicle dynamics parameters, which is either the same as or different from the at least one first vehicle dynamics parameter. Alternatively, the third parameter can be a vehicle control input, such as a throttle pedal position or a brake pressure.

The controller can be configured to calculate an additional confidence value of the first signal. The additional confidence value can be calculated in dependence on a fourth operating parameter of the vehicle. The fourth operating parameter could be one of said vehicle dynamics parameters, which is either the same as or different from the second vehicle dynamics parameter. Alternatively, the fourth operating parameter can be a vehicle control input, such as a throttle pedal position or a brake pressure.

The controller could be configured to calculate a second confidence value of the second signal. The first and second confidence values could be combined, for example by applying a weighting. The combined confidence value could be used to determine the cut-off frequency of the low-pass signal filter and the cut-off frequency of the high-pass signal filter.

According to a further aspect of the present invention there is provided a vehicle incorporating the apparatus described herein.

According to a further aspect of the present invention there is provided a method of estimating a vehicle state, the method comprising:

-   -   determining a first estimation of the vehicle state in         dependence on at least one first vehicle dynamics parameter;     -   determining a filter coefficient in dependence on a first         vehicle operating parameter;     -   setting an operating frequency of a first signal filter in         dependence on the determined filter coefficient and using the         first signal filter to filter the first estimation to generate a         first filtered estimation of the vehicle state; and     -   outputting a control signal in dependence on the first filtered         estimation of the vehicle state.

The vehicle state can be a pitch angle of the vehicle measured about a transverse axis. The at least one first vehicle dynamics parameter can comprise a reference velocity along a longitudinal axis of the vehicle.

The first signal filter can be a low-pass signal filter. The operating frequency of the first signal filter can be a cut-off frequency of the low-pass signal filter.

The method can comprise: determining a second estimation of the vehicle state in dependence on at least one second vehicle dynamics parameter; setting an operating frequency of a second signal filter in dependence on the determined filter coefficient and using the second signal filter to filter the second estimation to generate a second filtered estimation of the vehicle state. The second estimation can be determined by referencing the at least one second vehicle dynamics parameter to a look-up table stored in system memory.

The second signal filter can be a high-pass signal filter. The operating frequency of the second signal filter can be a cut-off frequency of the high-pass signal filter.

The method can comprise combining the first and second filtered estimations.

The second estimation of the vehicle state can determine a relative body pitch angle of the vehicle.

The method can comprise generating a confidence value of the first estimation in dependence on a first vehicle operating parameter. The first confidence value can be generated in dependence on an absolute value of the first vehicle operating parameter. The first confidence value can be proportional (either directly or inversely) to the first vehicle operating parameter. The first confidence value can be generated in dependence on the rate of change of the first vehicle operating parameter. The first confidence value can be proportional (either directly or inversely) to the rate of change of the first vehicle operating parameter. A filter can be applied to the determined rate of change of the first vehicle operating parameter. The filter coefficient can be calculated based on said confidence value.

The first vehicle operating parameter can comprise longitudinal vehicle acceleration. The method can comprise determining a rate of change of the longitudinal vehicle acceleration. The first confidence value of the first estimation can be generated in dependence on said determined rate of change of the longitudinal vehicle acceleration. A high frequency filter can be applied to the determined rate of change of the longitudinal vehicle acceleration.

The first vehicle operating parameter can comprise a throttle pedal position. The method can comprise determining a rate of change of the throttle pedal position to generate a second confidence value of the first estimation. A high frequency filter can be applied to the determined rate of change of the throttle pedal position.

The first vehicle operating parameter can comprise brake pressure. The method can comprise analysing the brake pressure to generate a third confidence value of the first estimation.

The first vehicle operating parameter can comprise at least one wheel slip measurement. The method can comprise analysing the at least one wheel slip measurement to generate a fourth confidence value of the first estimation. The at least one wheel slip measurement can be compared to a look-up table to generate the fourth confidence value. The analysis of the wheel slip measurement can comprise comparing first and second wheel slip measurements to a look-up table.

A filter coefficient can be generated in dependence on the generated confidence value, or in dependence on one of the generated confidence values. The filter coefficient can be generated in dependence on the generated confidence value indicating the lowest confidence in the accuracy of the first estimation. The method can comprise inverting the generated confidence value, the filter coefficient being generated in dependence on the inverted confidence value.

According to a further aspect of the present invention there is provided a dynamic filtering method comprising:

-   -   generating a first signal and a second signal;     -   calculating a cut-off frequency;     -   applying the calculated cut-off frequency to a low-pass signal         filter and filtering the first signal using the low-pass signal         filter;     -   applying the calculated cut-off frequency to a high-pass signal         filter and filtering the second signal using the high-pass         signal filter; and     -   combining the filtered outputs of said low-pass signal filter         and said high-pass signal filter. The cut-off frequency of the         low-pass signal filter and the cut-off frequency of the         high-pass signal filter are set at the same frequency to provide         complementary filtering of the first and second signals. The         method can comprise outputting a control signal in dependence on         the combined filtered outputs of said low-pass signal filter and         said high-pass signal filter.

The first signal can be generated in dependence on a first parameter. The second signal can be generated in dependence on a second parameter.

The method can comprise calculating a first confidence value of the first signal. The cut-off frequency can be calculated in dependence on said first confidence value. The first confidence value can be calculated in dependence on a third parameter.

According to a further aspect of the present invention there is provided a controller configured to perform the method(s) described herein. The controller can be configured to perform a set of computational instructions held in system memory. When executed, the computational instructions can cause the controller to perform the method(s) described herein. The controller can be a general purpose computational device or can be a dedicated computational device.

According to a further aspect of the present invention there is provided a machine-readable medium containing a set of computational instructions which, when executed, cause a controller to perform the method(s) described herein.

Any controller or controllers described herein may suitably comprise a control unit or computational device having one or more electronic processors. Thus the system may comprise a single control unit or electronic controller or alternatively different functions of the controller may be embodied in, or hosted in, different control units or controllers. As used herein the term “controller” or “control unit” will be understood to include both a single control unit or controller and a plurality of control units or controllers collectively operating to provide any stated control functionality. To configure a controller, a suitable set of instructions may be provided which, when executed, cause said control unit or computational device to implement the control techniques specified herein. The set of instructions may suitably be embedded in said one or more electronic processors. Alternatively, the set of instructions may be provided as software saved on one or more memory associated with said controller to be executed on said computational device. A first controller may be implemented in software run on one or more processors. One or more other controllers may be implemented in software run on one or more processors, optionally the same one or more processors as the first controller. Other suitable arrangements may also be used.

Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 shows a schematic overview of a vehicle incorporating a vehicle state estimation apparatus in accordance with an embodiment of the present invention;

FIG. 2 illustrates the pitch angles of a vehicle travelling on an inclined surface;

FIG. 3 represents the measurements taken by an inertial measurement unit provided on a vehicle;

FIG. 4 shows a flow diagram representing the operation of the global pitch angle estimator shown in FIG. 3;

FIG. 5 shows a schematic representation of the processor functions for the vehicle state estimation apparatus;

FIG. 6 illustrates the application of a variable low-pass signal filter and a variable high-pass signal filter to the unfiltered global pitch angle and the relative body pitch angle;

FIG. 7 illustrates the generation of a filter coefficient to control the variable low-pass signal filter and the variable high-pass signal filter shown in FIG. 6;

FIG. 8 shows a block diagram representing the operation of the processor to generate confidence values to determine the filter coefficient;

FIGS. 9A, B and C show a first set of measured vehicle parameters with respect to time;

FIGS. 10A and 10B show the dynamic filtering of pitch estimation based on the measured vehicle parameters shown in FIGS. 9A-C;

FIGS. 11A, B and C show a second set of measured vehicle parameters with respect to time;

FIGS. 12A and 12B show the dynamic filtering of pitch estimation based on the measured vehicle parameters shown in FIGS. 11A-C

FIG. 13 shows a schematic overview of a vehicle incorporating a vehicle reference velocity estimation apparatus in accordance with a further embodiment of the present invention;

FIG. 14 shows a schematic representing of the reference velocity for the vehicle shown in FIG. 13;

FIG. 15 shows a flow diagram representing determination of the longitudinal vehicle velocity estimate;

FIG. 16 shows a flow diagram representing determination of the reference velocity confidence estimator and output to a vehicle dynamic controller;

FIG. 17 shows a flow diagram representing dynamic filtering of the reference velocity in dependence on control inputs;

FIG. 18 shows a flow diagram representing dynamic high-pass and low-pass filtering of the vehicle reference velocity;

FIG. 19 shows a flow diagram representing operation of a side slip proportional derivative controller in dependence on a determined reference velocity confidence.

DETAILED DESCRIPTION

A vehicle state estimation apparatus 1 in the form of a global pitch angle estimator will now be described with reference to the accompanying Figures.

A schematic representation of a vehicle 3 incorporating the vehicle state estimation apparatus 1 is shown in FIG. 1. The vehicle 3 in the present embodiment is an automotive vehicle having four wheels FL, FR, RL, RR, an inertial measurement unit (IMU) 5, a throttle pedal 7, a brake pedal 9 and a steering wheel 11. A rotational speed sensor 13 is provided to measure the rotational speed of each wheel FL, FR, RL, RR to generate wheel speed signals WS1-4. The wheel speed signals WS1-4 are used to determine a reference velocity V of the vehicle 3 and, as described herein, to detect wheel spin. A first position sensor 15 is provided to measure the position of the throttle pedal 7 and to output a throttle pedal position signal S1. A pressure sensor 17 is provided to measure the hydraulic pressure in the brake system and to output a brake pressure signal S2. A steering wheel angle sensor 19 is provided to measure the steering angle θ of the steering wheel 11 and to output a steering angle signal S3.

The vehicle state is defined with reference to a longitudinal axis X, a transverse axis Y and a vertical axis Z of the vehicle 3. The reference speed V of the vehicle 3 is measured along the longitudinal axis X. As shown in FIG. 2, rotation about the longitudinal axis X is referred to as vehicle roll; rotation about the transverse axis Y is referred to as vehicle pitch; and rotation about the vertical axis Z is referred as vehicle yaw. The attitude of the vehicle 3 is defined by a roll angle θ_(x) (angular rotation about the longitudinal axis X), a pitch angle θ_(y) (angular rotation about the transverse axis Y) and a yaw angle θ_(z) (angular rotation about the vertical axis Z). The IMU 5 comprises accelerometers arranged to measure acceleration in six degrees of freedom, as illustrated in FIG. 3. The IMU 5 comprises an accelerometer adapted to measure the longitudinal acceleration A_(X) of the vehicle 3 (i.e. acceleration along the longitudinal axis X) and to output a longitudinal acceleration signal. Suitable IMUs are known in the art and described, for example IMU BMI055 produced by Bosch-Sensortec which measures six degrees of freedom.

The pitch angle θ_(y) of the vehicle 3 in relation to a horizontal axis and is referred to as the global pitch angle θ_(y). The global pitch angle θ_(Y) comprises a road pitch angle θ_(Y1) and a relative body pitch angle θ_(Y2). The road pitch angle θ_(Y1) corresponds to an incline angle of the road (or other surface on which the vehicle 3 is situated); and the relative body pitch angle θ_(Y2) corresponds to the pitch of the vehicle body relative to the road pitch angle θ_(Y1). The relative body pitch angle θ_(Y2) changes due to acceleration/deceleration forces and/or vehicle loads. The global pitch angle θ_(y) is used to estimate lateral kinematics and velocities, for example to determine a side slip angle of the vehicle 3.

As shown in FIG. 1, the vehicle state estimation apparatus 1 comprises a processor 21 coupled to system memory 23. The processor 21 is configured to perform a set of computational instructions held in the system memory 23. The processor 21 is in communication with a vehicle communication network, such as a controller area network (CAN) bus or FlexRay, to receive the wheel speed signals WS1-4, the longitudinal acceleration signal, the throttle pedal position signal S1, the brake pressure signal S2 and the steering angle signal S3.

The longitudinal acceleration signal output by the IMU 5 contains a component due to gravity and, under yaw conditions, a component from centripetal acceleration. These components may contaminate the longitudinal acceleration signal and result in errors. In order to determine the global pitch angle θ_(y) the vehicle pure longitudinal acceleration is determined from the reference velocity V. The reference velocity V is calculated from the wheel speed signals WS1-4, either by the processor 21 or a separate processor. In the present embodiment, the reference velocity V is calculated as the mean of the rotational speeds of the wheels FL, FR, RL, RR, however any other known methods of obtaining a reference velocity, for example the speed of the second slowest moving wheel or the average speed of two un-driven wheels of the vehicle, may of course be used. As will be understood the term reference velocity is a term used in the art to describe a speed of a vehicle derived from the speeds of two or more individual wheels speeds. Using the assumption that the vehicle 3 is in a condition of linear side slip, the estimated lateral velocity at the rear of the vehicle 3 can be translated to the position of the IMU 5. This assumption allows the global pitch angle θ_(y) to be calculated using the following global pitch estimation algorithm:

$\theta_{y} = {\sin^{- 1}\left( \frac{a_{x} - \overset{.}{u} + {\omega_{z} \cdot v_{y}}}{g} \right)}$

Where: θ_(y) is the global pitch angle;

-   -   a_(x) is the measured longitudinal acceleration;     -   {dot over (u)} is the derivative of the reference velocity V;     -   ω_(z) is the angular velocity about the Z axis;     -   v_(y) is the estimated lateral velocity; and     -   g is the acceleration due to gravity.

An overview of the operation of the vehicle state estimation apparatus 1 is provided in a first flow diagram 100 shown in FIG. 4. The vehicle state estimation apparatus 1 receives the measured longitudinal acceleration A_(X) and the reference velocity V (STEP 105). A first estimation of the global pitch angle θ_(y) is calculated (STEP 110) using the global pitch estimation algorithm. A variable frequency filter is applied to the calculated (raw) global pitch angle θ_(y) to remove noise or erroneous overshoots. An operating frequency of the variable frequency filter is determined based on a confidence value providing an indication of the confidence in the calculated global pitch angle θ_(y). A plurality of confidence values are calculated in dependence on at least one vehicle dynamic parameter and/or at least one control input which one of the confidence values is selected to determine a filter coefficient to control an operating frequency of the variable frequency filter. In the present embodiment, the at least one vehicle dynamic parameter comprises wheel spin which influences the accuracy of the determined reference velocity V; and the at least one control input received by the vehicle state estimation apparatus 1 comprises the throttle pedal position signal S1 and/or the brake pressure signal S2 (STEP 115). A check is performed to determine if the control input(s) is significant (STEP 120), for example to determine if a detected rate of change of the control input is greater than a defined threshold. If the vehicle state estimation apparatus 1 determines that the control inputs are not significant, no action is required (STEP 125). If the vehicle state estimation apparatus 1 determines that the control inputs are significant, the confidence value is modified dynamically. The determined confidence value sets the operating frequency of the variable frequency filter which is then used to filter the calculated global pitch angle θ_(y) (STEP 130). The filtered global pitch signal is output as a filtered global pitch angle estimate θ_(YF) (STEP 135) which can be used, for example, to implement vehicle dynamics control.

The processor 21 is represented schematically in FIGS. 5, 6 and 7. The processor 21 is configured to implement a reference velocity calculator 25, a global pitch calculator 27, a relative body pitch calculator 29, a slip calculator 31 and a confidence estimator 33. The processor 21 also implements a variable frequency low-pass signal filter 35 and a variable frequency high-pass signal filter 37. The cut-off frequency of the variable frequency low-pass signal filter 35 can be set at between zero (0) and one (1) Hertz inclusive. The cut-off frequency of the variable frequency high-pass signal filter 37 can be set between zero (0) and one (1) Hertz, inclusive. The cut-off frequency of the variable frequency low-pass signal filter 35 is set at the same value as the cut-off frequency of the variable frequency high-pass signal filter 37. It will be understood that the filter range could be changed for different applications. For example, a filter range of 0-0.7 Hz would be workable.

The reference velocity calculator 25 receives the wheel speed signals WS1-4 from the rotational speed sensors 13 associated with each wheel FL, FR, RL, RR. The reference velocity V is calculated using the wheel speed signals WS1-4 to determine the mean rotational speed WS of the wheels FL, FR, RL, RR. The reference velocity V is output to the global pitch calculator 27 and to the slip calculator 31. As outlined above, the global pitch calculator 27 uses the reference velocity V and the measured longitudinal acceleration A_(X) to calculate the global pitch angle θ_(y). The global pitch angle θ_(y) is output to the variable frequency low-pass signal filter 35. The relative body pitch calculator 29 uses the measured longitudinal acceleration A_(X) to determine the relative body pitch angle θ_(Y2). In particular, the relative body pitch calculator 29 cross-references the measured longitudinal acceleration A_(X) with empirically derived data stored in the system memory 23 to determine the relative body pitch angle θ_(Y2) commensurate with a measured longitudinal acceleration A_(X). For example, a stored value of pitch gradient can be referenced and multiplied by the measured longitudinal acceleration A_(X). The relative body pitch angle θ_(Y2) is output to the variable frequency high-pass signal filter 37.

The slip calculator 31 compares the wheel speed signals WS1-4 to the reference velocity V to determine the wheel slip for each wheel FL, FR, RL, RR. The confidence estimator 33 receives the calculated wheel slip for each wheel FL, FR, RL, RR, along with the measured longitudinal acceleration A_(X), the brake pressure signal S2 and the throttle pedal position signal S1. The confidence estimator 33 calculates a confidence value F in the calculated global pitch angle θ_(y). In the present embodiment, the confidence value F lies in the range zero (0) to one (1), with zero (0) representing the maximum confidence and one (1) representing the minimum confidence. The confidence value F is used to determine a filter coefficient F_(C) to set the cut-off frequency of the variable frequency low-pass signal filter 35 and the cut-off frequency of the variable frequency high-pass signal filter 37. The cut-off frequency of the variable frequency low-pass signal filter 35 is set at the same value as the cut-off frequency of the variable frequency high-pass signal filter 37 to provide complementary signal filtering. In the present embodiment, the filter coefficient F_(C) is calculated by subtracting the determined confidence value F from one (1). Thus, the smaller the confidence value F (representing a higher confidence in the calculated global pitch angle θ_(y)), the higher the cut-off frequency of the variable frequency low-pass signal filter 35 and the variable frequency high-pass signal filter 37. Conversely, the larger the confidence value F (representing a lower confidence in the calculated global pitch angle θ_(y)), the lower the cut-off frequency of the variable frequency low-pass signal filter 35 and the variable frequency high-pass signal filter 37. As illustrated in FIG. 6, the global pitch angle θ_(y) is filtered by the variable frequency low-pass signal filter 35; and the relative body pitch angle θ_(Y2) is filtered by the variable frequency high-pass signal filter 37. The processor 21 sums the filtered signals to generate the filtered global pitch angle estimate θ_(YF).

The operation of the confidence estimator 33 will now be described in more detail with reference to a block diagram 200 shown in FIG. 8.

The confidence estimator 33 receives the measured longitudinal acceleration A_(X) and determines a rate of change of the longitudinal acceleration A_(X) with respect to time (STEP 205), which can be referred to as jerk. A first discrete high frequency filter (for example 3-5 Hz) is applied to the rate of change signal (STEP 210) and the magnitude of the resultant signal determined (STEP 215). A first gain K1 is then applied (STEP 220) to generate a first confidence value F1 which provides an indication of a confidence in the calculated global pitch angle θ_(y) based on the current rate of change in the longitudinal acceleration A_(X) of the vehicle 3. In the present embodiment, the first gain K1 is set at 0.08, but this value can be calibrated to suit particular applications. The first confidence value F1 is output to a comparator 39.

The confidence estimator 33 receives the throttle pedal position signal S1 and determines a rate of change of the throttle pedal position with respect to time (STEP 225). A second discrete high frequency filter (for example 5 Hz) is applied to the rate of change signal (STEP 230) and the magnitude of the resultant signal determined (STEP 235). A second gain K2 is applied (STEP 240) to generate a second confidence value F1 which provides an indication of a confidence in the calculated global pitch angle θ_(y) based on the current rate of change of the throttle pedal position. In the present embodiment, the second gain K1 is set at 0.003, but this value can be calibrated to suit particular applications. The second confidence value F2 is output to the comparator 39.

The confidence estimator 33 receives the brake pressure signal S2 and determines the magnitude of the brake pressure (STEP 245). The brake pressure is compared to a look-up table (STEP 250) to generate a third confidence value F3 which provides an indication of a confidence in the calculated global pitch angle θ_(y) based on the current the brake pressure. The look-up table defines a dead band for brake pressures below 50 bar. If the brake pressure is less than 50 bar, a value of zero (0) is returned as a third confidence value F3. If the brake pressure is greater than 50 bar, a third gain K3 is applied to generate the third candidate filter coefficient F3. In the present embodiment, the third gain K3 is interpolated linearly between 0 and 1 in dependence on brake pressure measurement between 50 bar and 100 bar. By way of example, the third gain K3 is set as 1 when the brake pressure is greater than or equal to 100 bar, 0.5 when the brake pressure is 75 bar; and zero when the brake pressure is less than or equal to 50 bar. The third confidence value F3 provides an indication of a confidence in the calculated global pitch angle θ_(y) based on the current brake pressure. It will be understood that the third gain K3 can be calibrated to suit particular applications.

The slip calculator 31 receives the wheel speed signals WS1-4 from each speed sensor to determine the difference in the rotational speed of the front and rear wheels on each side of the vehicle. A first slip calculator 41 receives the wheel speed signals WS1, S3 for the wheels FL, RL on the left hand side of the vehicle 3 and determines the difference in their respective rotational speeds (STEP 255). The first slip calculator 41 subtracts the rotational speed of the rear left wheel RL from the rotational speed of the front left wheel FL and outputs a first slip value SL1. The first slip value SL1 is output to a first low-pass signal filter 43 which filters the first slip value SL1 (STEP 260) and the first filtered slip value SL1F is output to a multiplexer 45. The first filtered slip value SL1F is expressed as a percentage (%).

A second slip calculator 47 receives the wheel speed signals S2, S4 for the wheels FR, RR on the right hand side of the vehicle 3 and determines the difference in their respective rotational speeds (STEP 265). The second slip calculator 47 subtracts the rotational speed of the rear right wheel RL from the rotational speed of the front right wheel FR and outputs a second slip value SL2. The second slip value SL2 is output to a second low-pass signal filter 49 which filters the second slip value SL2 (STEP 270) and the second filtered slip value SL2F is output to the multiplexer 45. The second filtered slip value SL2F is expressed as a percentage (%).

The multiplexer 45 outputs an array comprising the first and second filtered slip values SL1F, SL2F (STEP 275). A fourth gain K4 is applied to the array (STEP 280) to generate a fourth confidence value F4. The fourth gain K4 is a non-linear relationship defined with reference to a graph in which the fourth gain K4 is defined along an X-axis (0, 0, 0.3. 0.8, 0.9) and the wheel slip is defined along a Y-axis (0, 0.008, 0.01, 0.015, 0.035). The fourth confidence value F4 provides an indication of a confidence in the calculated global pitch angle θ_(y) based on the detected wheel slip. By way of example, a detected wheel slip of 1% results in the fourth confidence value F4 being output as 0.3. The maximum detected wheel slip SL1F, SL2F is compared to a predetermined slip threshold (STEP 285), the slip threshold being set as 4.5% in the present embodiment. An uncertainty signal S5 is output to indicate a confidence rating in the calculated global pitch angle θ_(y). The uncertainty signal S5 is set to zero (0) if the detected wheel slip exceeds the slip threshold; and the uncertainty signal is set to one (1) if the detected wheel slip is less than the slip threshold.

The first, second, third and fourth gains K1-4 are operative to normalize the first, second, third and fourth confidence values F1-4 to one (1), such that zero (0) represents the lowest confidence and one (1) represents the highest confidence. The comparator 39 selects the highest of the first, second, third and fourth confidence values F1-4 which represents the lowest confidence in the calculated global pitch angle θ_(y) (STEP 290). The processor 21 subtracts the selected confidence value Fx from one (1) (STEP 295) and applies upper and lower saturation limits (STEP 300). The upper and lower saturation limits are set as 0.01 and 1 respectively. The resulting signal is multiplied by the uncertainty signal S5 (STEP 305) and a rising rate limit applied (STEP 310). In the present embodiment, the rising rate limit is set to 0.7. The resulting signal is output (STEP 315) from the confidence estimator 33 as a dynamic filter coefficient F_(C). The dynamic filter coefficient F_(C) sets the first cut-off frequency of the variable frequency low-pass signal filter 35 and the second cut-off frequency of the variable frequency high-pass signal filter 37.

The calculated global pitch angle θ_(Y) is filtered by the variable frequency low-pass signal filter 35; and the relative body pitch angle θ_(Y2) is filtered by the variable frequency high-pass signal filter 37. The processor 21 sums the filtered signals to generate the filtered global pitch angle estimate θ_(YF) for output from the vehicle state estimation apparatus 1. As described herein, the filtered global pitch angle estimate θ_(YF) can be used by vehicle dynamics controls.

The operation of the vehicle state estimation apparatus 1 to generate the filtered global pitch angle estimate θ_(YF) will now be described for a first dynamic scenario in which the vehicle 3 undergoes heavy braking from a reference velocity of approximately 100 kph to 5 kph. FIG. 9A shows a first graph 400 showing a measured steering wheel angle)(°); FIG. 9B shows a second graph 410 showing a measured reference velocity (U); and FIG. 9C shows a third graph 420 showing a measured lateral acceleration (g). The steering wheel angle, reference velocity and the lateral acceleration are measured concurrently during a first time period. The dynamic filtering of the calculated global pitch angle θ_(Y) during the first time period is represented in a fourth graph 430 shown in FIG. 10A; and the determined dynamic filter coefficient F_(C) during the first time period is represented in a fifth graph 440 shown in FIG. 10B. With reference to FIG. 10A, a first trace T1 shows the calculated global pitch angle θ_(Y); a second trace T2 shows the filtered global pitch angle estimate θ_(YF) generated when the dynamic filter is applied to the calculated global pitch angle θ_(Y); a third trace T3 shows a filtered global pitch angle θ_(Y) generated by applying a discrete filter to the calculated global pitch angle θ_(Y); and a fourth trace T4 shows a measured global pitch angle θ_(Y) for comparison purposes. The calculated global pitch angle θ_(Y) creates an erroneous signal in periods of wheel slip during the braking event. However, by dynamically controlling the filter coefficient F_(C), the filtered global pitch angle estimate θ_(YF) more closely follows the measured global pitch angle θ_(Y). This is evident from FIG. 10A in which the second trace T2 more closely follows the fourth trace T4 than either the first trace T1 or the third trace T3.

The operation of the vehicle state estimation apparatus 1 to generate the filtered global pitch angle estimate θ_(YF) will now be described for a second dynamic scenario in which the vehicle 3 experiences excess roll as it travels around a hairpin corner. FIG. 11A shows a sixth graph 450 showing a measured steering wheel angle)(°); FIG. 11B shows a seventh graph 460 showing a measured reference velocity (U); and FIG. 11C shows an eighth graph 470 showing a measured lateral acceleration (g). The steering wheel angle, reference velocity and the lateral acceleration are measured concurrently during a second time period. The dynamic filtering of the calculated global pitch angle θ_(Y) during the second time period is illustrated in a ninth graph 480 shown in FIG. 12A; and the determined dynamic filter coefficient F_(C) during the first time period is shown in a tenth graph 490 FIG. 12B. With reference to FIG. 11A, a first trace T1 shows the calculated global pitch angle θ_(Y); a second trace T2 shows the filtered global pitch angle estimate θ_(YF) generated when the dynamic filter is applied to the calculated global pitch angle θ_(Y); a third trace T3 shows a filtered global pitch angle θ_(Y) generated by applying a discrete filter to the calculated global pitch angle θ_(Y); and a fourth trace T4 shows a measured global pitch angle θ_(Y) for comparison purposes. The calculated global pitch angle θ_(Y) creates an erroneous signal in periods of wheel slip during the braking event. However, by dynamically controlling the filter coefficient F_(C), the filtered global pitch angle estimate θ_(YF) more closely follows the measured global pitch angle θ_(Y). This is evident from FIG. 11A in which the second trace T2 more closely follows the fourth trace T4 than either the first trace T1 or the third trace T3. The filter coefficient F_(C) during the second time period is shown in a fifth trace T5 in tenth graph 490 shown in FIG. 12B.

It will be appreciated that various changes and modifications can be made to the vehicle state estimation apparatus 1 described herein. The vehicle state estimation apparatus 1 could be configured to estimate body roll angle θ_(X). For example, the vehicle state estimation apparatus 1 could use dynamic vehicle parameters such as lateral velocity and/or lateral acceleration; and/or control inputs such as steering angle θ.

The vehicle state estimation apparatus 1 has been described with reference to determining the filtered global pitch angle estimate θ_(YF). However, it has been recognised that the techniques are also applicable to determine the reference velocity V of the vehicle 2. Notably, the confidence estimator 33 can provide an indication of the confidence in the reference velocity V. The dynamic filter coefficient F_(C) generated by the confidence estimator 33 can be used to set a cut-off frequency of a variable frequency low-pass signal filter 35 and/or a variable frequency high-pass signal filter 37. The reference velocity V can be determined in dependence on the resulting filtered signal(s). The reference velocity V is output to vehicle dynamic controllers and used to control dynamic operation of the vehicle 2. By determining confidence in the calculated reference velocity V and/or improving the accuracy of the reference velocity V, more robust vehicle control can be achieved. The application of the global pitch angle estimate techniques to determine the reference velocity V of the vehicle 2 will now be described with reference to FIGS. 13 to 18.

As shown in FIG. 13, the vehicle 3 is an automotive vehicle having four wheels FL, FR, RL, RR, an inertial measurement unit (IMU) 5, a throttle pedal 7, a brake pedal 9 and a steering wheel 11. The IMU 5 comprises a first accelerometer adapted to measure the longitudinal acceleration A_(X) of the vehicle 3 (i.e. acceleration along the longitudinal axis X); and a second accelerometer adapted to measure angular acceleration ω_(Z) about the vertical axis Z. A rotational speed sensor 13 is provided to measure the rotational speed of each wheel FL, FR, RL, RR to generate wheel speed signals WS1-4. A first position sensor 15 is provided to measure the position of the throttle pedal 7 and to output a throttle pedal position signal S1. A pressure sensor 17 is provided to measure the hydraulic pressure in the brake system and to output a brake pressure signal S2. A steering wheel angle sensor 19 is provided to measure the steering angle θ of the steering wheel 11 and to output a steering angle signal S3.

The rotational speed sensor 13 for each wheel FL, FR, RL, RR in the present embodiment is in the form of a magnetic (Hall effect) sensor operative in combination with a coded toothed toning disc in the associated wheel hub. The wheel speed is translated to a single datum point of the vehicle, for example to an assumed centre of gravity (CoG) of the vehicle 3. With reference to FIG. 14, the longitudinal wheel speed V of each wheel FL, FR, RL, RR, translated to the COG, is calculated using the following equations:

$V_{{FL},{CoG}} = {\frac{V_{FL}}{\cos \; \theta} + {{1/2}\; T\; \omega_{z}}}$ $V_{{FR},{CoG}} = {\frac{V_{FR}}{\cos \; \theta} - {{1/2}\; T\; \omega_{z}}}$ V_(RL, CoG) = V_(RL) + 1/2 T ω_(z) V_(RR, CoG) = V_(RR) − 1/2 T ω_(z)

Where: V_(CoG) is the wheel speed translated to the CoG;

-   -   V is the measured speed of each wheel (FL, FR, RL, RR);     -   θ is the steering angle;     -   ω_(Z) is the angular acceleration about the vertical axis Z; and     -   T is the wheel track.

As shown in FIG. 15, the measured wheel speeds V_(FL), V_(FR), V_(RL), V_(RR), the steering angle θ, the wheel track T, the angular acceleration ω_(Z) and the longitudinal acceleration A_(X) are output to a wheel speed translator 53 configured to calculate the translated longitudinal wheel speed V_(CoG) of each wheel FL, FR, RL, RR. The translated longitudinal wheel speeds V_(CoG) are output to the reference velocity estimator 55 which outputs the longitudinal reference velocity V for the vehicle 3.

The reference velocity V of the vehicle 3 can be determined by averaging the measured speed of each wheel FL, FR, RL, RR. The reference velocity estimator 55 can optionally perform one or more of the following functions:

-   -   (a) Remove effects of steering angle θ and/or yaw angle using         measured vehicle parameters from the steering wheel 11 and/or         dynamic parameters measured by the on board IMU 5.     -   (b) In a two-wheel drive application, the forward velocity can         be determined based on the measured speed of the non-driven         wheels (since these are less likely to be in positive slip from         positive engine torque).     -   (c) In high lateral acceleration maneuvers, the forward velocity         can be determined based on the average of the rotational speeds         of the outside wheels (since these are less likely to lose         traction with the road surface, for example due to lifting).     -   (d) Using longitudinal acceleration A_(X) from the IMU 5 to         perform plausibility checks on wheel speed information, for         example if the vehicle 3 is not decelerating but wheels speeds         are very low this can imply a wheel lock scenario (and one or         more measured wheel speeds can be ignored).     -   (e) Integration of longitudinal acceleration for short periods         of time when all wheel speeds are determined unstable.

A slip calculator 31 is provided for calculating wheel slip values SL1-4, as shown in FIG. 16. The slip calculator 31 uses the measured wheel speeds V_(FL), V_(FR), V_(RL), V_(RR) from the speed sensors 13 to calculate the wheel slip values, as described herein with reference to the vehicle state estimation apparatus 1. A reference velocity confidence estimator 157 is provided to calculate one or more confidence values F1 in dependence on at least one vehicle dynamic parameter and/or at least one control input. The calculated confidence value F1 provides an indication of the confidence in the calculated reference velocity V. In the present embodiment the confidence value F1 ranges from 0 to 1 (inclusive), where 0 represents low confidence and 1 represents high confidence. The reference velocity confidence estimator 157 receives at least one vehicle dynamic parameter such as the wheel slip values SL1-4 determined by the slip calculator 31 and/or longitudinal acceleration measured by the IMU 5; and at least one control input, such as the throttle pedal signal S1 and/or the brake pressure signal S2. The reference velocity confidence estimator 157 can optionally also receive the measured wheel speeds V_(FL), V_(FR), V_(RL), V_(RR), the steering angle θ, the wheel track T, the angular acceleration ω_(Z) and the longitudinal acceleration A_(X) from the corresponding sensors. The confidence value F1 is output to the reference velocity estimator 55 and/or to a dynamic controller 59. The reference velocity estimator 55 determines the vehicle reference velocity V in dependence on the confidence value F1, for example by applying a dynamic filter.

An overview of the operation of the reference velocity confidence estimator 57 is provided in a flow diagram 500 shown in FIG. 17. The reference velocity confidence estimator 57 receives the wheel speeds V_(FL), V_(FR), V_(RL), V_(RR), (STEP 505), which can be filtered or unfiltered (raw) data. A first estimation of the reference velocity V is calculated (STEP 510) using the wheel speeds V_(FL), V_(FR), V_(RL), V_(RR). A variable frequency filter is applied to the calculated reference velocity V (STEP 515) to remove noise or erroneous overshoots. An operating frequency of the variable frequency filter is determined in dependence on the calculated confidence value F1 (or a selected one of a plurality of said calculated confidence values F1). Specifically, the calculated confidence value F1 is used to determine a filter coefficient F_(C) to control an operating frequency of the variable frequency filter. In the present embodiment, the confidence value F1 is calculated in dependence on a control input in the form of the throttle pedal position signal S1 and/or the brake pressure signal S2 (STEP 520). The reference velocity confidence estimator 57 receives the at least one control input (STEP 525) and performs a check to determine if the control inputs are significant (STEP 530). For example, the reference velocity confidence estimator 57 can determine if the throttle pedal position signal S1 and the brake pressure signal S2 exceed respective thresholds. If the control inputs are determined not to be significant, no action is required (STEP 535). If, however, the control inputs are determined to be significant, the confidence value F1 is modified dynamically. The determined confidence value F1 sets the operating frequency of the variable frequency filter which is then used to filter the calculated reference velocity V (STEP 540). A filtered reference velocity V_(F) is output (STEP 535), for example to a vehicle dynamics controller to control dynamic operation of the vehicle 3.

In a similar manner to the dynamic filtering of global and relative pitch described herein, the determined confidence value F1 can be used to calculate a filtered reference velocity V_(F) from multiple reference velocity sources. By generating the filtered reference velocity from several different sources, a more accurate estimate of the reference velocity V can be obtained. A first reference velocity V₁ can be derived from the measured speed of the wheels FL, FR, RL, RR; and a second reference velocity V₂ can be derived from a second source, such as integration of longitudinal acceleration A_(X) from the IMU 5, the speed obtained from GPS information, or another source. The first and second reference velocities V₁, V₂ can be dynamically filtered in dependence on the determined confidence value F1 and then combined to generate the filtered reference velocity V_(F). The two filtered signals can complement each other to cover the whole desired frequency range. Indeed, at least in certain embodiments, there may be a third or additional source(s) of reference velocity V and a three way or more combination of signals made. The calculation of a filtered reference velocity V_(F) from multiple sources will now be described.

The determination of the filtered reference velocity V_(F) from first and second reference velocities V₁, V₂ will now be described with reference to in a flow diagram 600 shown in FIG. 18. The first reference velocity V₁ is obtained from a first source which in the present embodiment is the reference velocity calculator 25 which receives the wheel speed signals WS1-4 from the rotational speed sensors 13 associated with each wheel FL, FR, RL, RR. The first reference velocity V₁ is calculated using the wheel speed signals WS1-4 to determine the mean rotational speed WS of the wheels FL, FR, RL, RR (STEP 605). The second reference velocity V₂ is obtained from a second source which in the present embodiment is the longitudinal acceleration A_(X) measured by the IMU 5 (STEP 610). Alternatively, or in addition, the second source could comprise global positioning system (GPS) operative to measure the second reference velocity V₂.

The first reference velocity V₁ is output to the variable frequency low-pass filter 35; and the second reference velocity V₂ is output to the variable frequency high-pass filter 37. A cut-off frequency of the variable frequency low-pass signal filter 35 can be set at between zero (0) and one (1) Hertz inclusive. Similarly, the cut-off frequency of the variable frequency high-pass signal filter 37 can be set between zero (0) and one (1) Hertz inclusive. As described herein, the reference velocity confidence estimator 57 calculates the confidence value F1 in dependence on at least one vehicle dynamic parameter and/or at least one control input. In the present embodiment, the confidence value F lies in the range zero (0) to one (1), with zero (0) representing the maximum confidence and one (1) representing the minimum confidence. The confidence value F is used to determine a filter coefficient F_(C) to set the cut-off frequency of the variable frequency low-pass signal filter 35 and the cut-off frequency of the variable frequency high-pass signal filter 37. The cut-off frequency of the variable frequency low-pass signal filter 35 and the variable frequency high-pass signal filter 37 are set at the same value in dependence on the determined filter coefficient F_(C). As illustrated in FIG. 18, the first reference velocity V₁ is filtered by the variable frequency low-pass filter 35 (STEP 615); and the second reference velocity V₂ is filtered by the variable frequency high-pass filter 37 (STEP 620). The filtered first and second signals are then summed (STEP 625) to generate the filtered global reference velocity V_(F). The filtered global reference velocity V_(F) is then output (STEP 630).

Alternatively, or in addition, the reference velocity confidence estimator 57 can be output to a vehicle dynamic controller 61. The use of the reference velocity confidence estimator 57 to control the vehicle dynamic controller 61 is illustrated in a flow diagram 700 shown in FIG. 19. A reference longitudinal velocity V_(x) (STEP 705) and a reference transverse velocity V_(y) (STEP 710) are input to a side slip estimator 163 which estimates a side slip angle β at the rear axle using the equation β=V_(y)/V_(x) (STEP 715). A side slip angle β (STEP 720) and a side slip rate {dot over (β)} (STEP 725) are output to a proportional derivative (PD) side slip controller 163 (STEP 730). The reference velocity confidence estimator 157 determines the confidence value F1 for the calculated longitudinal reference velocity V_(x) and/or the calculated transverse reference velocity V_(Y) and this is also output to the PD side slip controller 163 (STEP 735). The PD side slip controller 163 outputs a control signal to the vehicle dynamic controller 61 (STEP 740). The operation of the vehicle dynamic controller 61 is controlled in dependence on the control signal. By way of example, if the confidence value F1 output to the PD side slip controller 163 is low, this can provide an indication that the calculated side slip angle β is smaller than or larger than was intended. The PD side slip controller 163 is configured to output a control signal to the vehicle dynamic control to deliver brake pressures to the wheels to generate a yaw torque. The PD side slip controller 61 can be tuned to achieve desired levels of vehicle slip angle or rate magnitude. In the event that the reference velocity confidence estimator 157 determining a low confidence in the calculated reference velocity V, the PD side slip controller 163 could be configured to turn off the PD side slip controller 163, as response cannot be relied upon. Alternatively, or in addition, the PD side slip controller 163 can be configured to change the vehicle dynamic controller 61 to an alternate set of tuneable parameters, for example switch to a “sensitised” control setting having tighter dead bands and/or gains in order to capture side slip events at lower input levels. At least in certain embodiments, this approach would be appropriate since the calculated reference velocity V might be greater than the real value.

It will be appreciated that various changes and modifications can be made to the apparatus and methods described herein without departing from the scope of the present application.

Further aspects of the present invention are set out in the following numbered paragraphs:

1. An apparatus for estimation of a vehicle state, the apparatus comprising

-   -   a controller comprising an electronic processor having an         electrical input for receiving at least one first vehicle         dynamics parameter signal and a least a first vehicle operating         parameter signal;     -   an electronic memory device electrically coupled to the         electronic processor and having instructions stored therein,     -   wherein the electronic processor is configured to access the         memory device and execute the instructions stored therein such         that it is operable to:         -   determine a first estimation of the vehicle state in             dependence on at least one first vehicle dynamics parameter;         -   determine a filter coefficient in dependence on a first             vehicle operating parameter; and         -   set an operating frequency of a first signal filter in             dependence on the determined filter coefficient and use the             first signal filter to filter the first estimation to             generate a first filtered estimation of the vehicle state;             and         -   output a control signal in dependence on the first filtered             estimation of the vehicle state.             2. An apparatus as described in paragraph 1, wherein the             vehicle state is a pitch angle of the vehicle measured about             a transverse axis; and the at least one first vehicle             dynamics parameter comprises a reference velocity along a             longitudinal axis of the vehicle.             3. An apparatus as described in paragraph 1, wherein said             first signal filter is a low-pass signal filter and the             operating frequency of the first signal filter is a cut-off             frequency of the low-pass signal filter.             4. Apparatus as described in paragraph 1, wherein the at             least one first vehicle dynamics parameter is one or more             parameters selected from the following set: reference             velocity, longitudinal velocity, longitudinal acceleration,             lateral velocity, lateral acceleration, vertical velocity,             vertical acceleration, roll, yaw, pitch and wheel slip.             5. An apparatus as described in paragraph 1, wherein the             electronic processor is configured to access the memory             device and execute the instructions stored therein such that             it is operable to:     -   determine a second estimation of the vehicle state in dependence         on at least one second vehicle dynamics parameter;     -   set an operating frequency of a second signal filter in         dependence on the determined filter coefficient and use the         second signal filter to filter the second estimation to generate         a second filtered estimation of the vehicle state.         6. An apparatus as described in paragraph 5, wherein the second         estimation is determined by referencing the at least one second         vehicle dynamics parameter to a look-up table stored in system         memory.         7. Apparatus as described in paragraph 5, wherein the at least         one second vehicle dynamics parameter is one or more parameters         selected from the following set: reference velocity,         longitudinal velocity, longitudinal acceleration, lateral         velocity, lateral acceleration, vertical velocity, vertical         acceleration, roll, yaw, pitch, and wheel slip.         8. An apparatus as described in paragraph 5, wherein said second         signal filter is a high-pass signal filter, and the operating         frequency of the second signal filter is a cut-off frequency of         the high-pass signal filter.         9. An apparatus as described in paragraph 5, wherein the         electronic processor is operable to combine the first and second         filtered estimations.         10. An apparatus as described in paragraph 5, wherein the second         estimation of the vehicle state determines a relative body pitch         angle of the vehicle.         11. An apparatus as described in paragraph 1, wherein the         wherein the electronic processor is operable to access the         memory device and execute the instructions stored therein such         that it is operable to:     -   generate a confidence value of the first estimation in         dependence on the first vehicle operating parameter; and     -   the filter coefficient is calculated based on said confidence         value.         12. An apparatus as described in paragraph 11, wherein said         first vehicle operating parameter comprises longitudinal vehicle         acceleration; and the electronic processor is operable to         determine a rate of change of the longitudinal vehicle         acceleration and to generate a first confidence value of the         first estimation in dependence on said determined rate of change         of the longitudinal vehicle acceleration.         13. An apparatus as described in paragraph 12, wherein the         electronic processor is operable to apply a high frequency         filter to the determined rate of change of the longitudinal         vehicle acceleration.         14. An apparatus as described in paragraph 11, wherein said         first vehicle operating parameter comprises a throttle pedal         position; and the electronic processor is operable to determine         a rate of change of the throttle pedal position and to generate         a second confidence value of the first estimation in dependence         on the determined rate of change of the throttle pedal position.         15. An apparatus as described in paragraph 14, wherein the         electronic processor is operable to apply a high frequency         filter to the determined rate of change of the throttle pedal         position.         16. An apparatus as described in paragraph 11, wherein said         first vehicle operating parameter comprises brake pressure; and         wherein the electronic processor is operable to generate a third         confidence value of the first estimation in dependence on the         brake pressure.         17. An apparatus as described in paragraph 11, wherein said         first vehicle operating parameter comprises at least one wheel         slip measurement; and wherein the electronic processor is         operable to generate a fourth confidence value of the first         estimation in dependence on the at least one wheel slip         measurement.         18. An apparatus as described in paragraph 17, wherein the at         least one wheel slip measurement is compared to a look-up table         to generate the fourth confidence value.         19. An apparatus as described in paragraph 17, wherein the         analysis of the wheel slip measurement comprises comparing first         and second wheel slip measurements to a look-up table.         20. An apparatus as described in paragraph 11, wherein the         filter coefficient is determined in dependence on the generated         confidence value.         21. An apparatus as described in paragraph 11, wherein the         electronic processor is operable to generate a plurality of said         confidence values, each confidence value being generated in         dependence on a different first operating parameter; and wherein         the controller is operable to generate the filter coefficient in         dependence on the generated confidence value indicating the         lowest confidence in the first estimation.         22. An apparatus as described in paragraph 20, wherein the         electronic processor is operable to invert the generated         confidence value, the filter coefficient being generated in         dependence on the inverted confidence value.         23. A dynamic filtering apparatus comprising:     -   a controller comprising an electronic processor having an         electrical input for receiving at least one first vehicle         dynamics parameter signal and a least a first vehicle operating         parameter signal;     -   an electronic memory device electrically coupled to the         electronic processor and having instructions stored therein,     -   wherein the electronic processor is configured to access the         memory device and execute the instructions stored therein such         that it is operable to:         -   generate a first signal and a second signal;         -   calculate a cut-off frequency;         -   apply the calculated cut-off frequency to a low-pass signal             filter and filter the first signal using the low-pass signal             filter;         -   apply the calculated cut-off frequency to a high-pass signal             filter and filter the second signal using the high-pass             signal filter; and         -   combine the filtered outputs of said low-pass signal filter             and said high-pass signal filter.             24. A dynamic filtering apparatus as described in paragraph             23, wherein the electronic processor is operable to generate             the first signal in dependence on at least one first             parameter.             25. A dynamic filtering apparatus as described in paragraph             24, wherein the at least one first parameter is at least one             first vehicle dynamics parameter selected from the following             set: reference velocity, longitudinal velocity, longitudinal             acceleration, lateral velocity, lateral acceleration,             vertical velocity, vertical acceleration, roll, yaw, pitch             and wheel slip.             26. A dynamic filtering apparatus as described in paragraph             23, wherein the electronic processor is operable to generate             the second signal in dependence on at least one second             parameter.             27. A dynamic filtering apparatus as claimed in claim 26,             wherein the at least one second parameter is at least one             second vehicle dynamics parameter selected from the             following set: reference velocity, longitudinal velocity,             longitudinal acceleration, lateral velocity, lateral             acceleration, vertical velocity, vertical acceleration,             roll, yaw, pitch, and wheel slip.             28. A dynamic filtering apparatus as described in paragraph             23, wherein the electronic processor is operable to             calculate a first confidence value of the first signal; and             the cut-off frequency is calculated in dependence on said             first confidence value.             29. A dynamic filtering apparatus as described in paragraph             28, wherein the first confidence value is calculated in             dependence on a third parameter.             30. A dynamic filtering apparatus as described in paragraph             29, wherein the third parameter is a vehicle dynamics             parameter or a vehicle control input.             31. A vehicle comprising apparatus as described in paragraph             1.             32. A method of estimating a vehicle state, the method             comprising:     -   determining a first estimation of the vehicle state in         dependence on at least one first vehicle dynamics parameter;     -   determining a filter coefficient in dependence on a first         vehicle operating parameter;     -   setting an operating frequency of a first signal filter in         dependence on the determined filter coefficient and using the         first signal filter to filter the first estimation to generate a         first filtered estimation of the vehicle state; and     -   outputting a control signal in dependence on the first filtered         estimation of the vehicle state.         33. A method as described in paragraph 32, wherein the vehicle         state is a pitch angle of the vehicle measured about a         transverse axis; and the at least one first vehicle dynamics         parameter comprises a reference velocity along a longitudinal         axis of the vehicle.         34. A method as described in paragraph 32, wherein said first         signal filter is a low-pass signal filter and the operating         frequency of the first signal filter is a cut-off frequency of         the low-pass signal filter.         35. A method as described in paragraph 32, wherein the at least         one first vehicle dynamics parameter is one or more parameters         selected from the following set: reference velocity,         longitudinal velocity, longitudinal acceleration, lateral         velocity, lateral acceleration, vertical velocity, vertical         acceleration, roll, yaw, pitch and wheel slip.         36. A method as described in paragraph 32 comprising:     -   determining a second estimation of the vehicle state in         dependence on at least one second vehicle dynamics parameter;     -   setting an operating frequency of a second signal filter in         dependence on the determined filter coefficient and using the         second signal filter to filter the second estimation to generate         a second filtered estimation of the vehicle state.         37. A method as described in paragraph 36 comprising determining         the second estimation by referencing the at least one second         vehicle dynamics parameter to a look-up table stored in system         memory.         38. A method as described in paragraph 36, wherein the at least         one second vehicle dynamics parameter is one or more parameters         selected from the following set: reference velocity,         longitudinal velocity, longitudinal acceleration, lateral         velocity, lateral acceleration, vertical velocity, vertical         acceleration, roll, yaw, pitch, and wheel slip.         39. A method as described in paragraph 36, wherein said second         signal filter is a high-pass signal filter, and the operating         frequency of the second signal filter is a cut-off frequency of         the high-pass signal filter.         40. A method as described in paragraph 36 comprising combining         the first and second filtered estimations.         41. A method as described in paragraph 36, wherein the second         estimation of the vehicle state determines a relative body pitch         angle of the vehicle.         42. A method as described in paragraph 32 comprising generating         a confidence value of the first estimation in dependence on a         first vehicle operating parameter; and calculating the filter         coefficient based on said confidence value.         43. A method as described in paragraph 42, wherein said first         vehicle operating parameter comprises longitudinal vehicle         acceleration; and the method comprises determining a rate of         change of the longitudinal vehicle acceleration, and generating         a first confidence value of the first estimation in dependence         on said determined rate of change of the longitudinal vehicle         acceleration.         44. A method as described in paragraph 43 comprising applying a         high frequency filter to the determined rate of change of the         longitudinal vehicle acceleration.         45. A method as described in paragraph 42, wherein said first         vehicle operating parameter comprises a throttle pedal position;         and the method comprises determining a rate of change of the         throttle pedal position to generate a second confidence value of         the first estimation.         46. A method as described in paragraph 45 comprising applying a         high frequency filter to the determined rate of change of the         throttle pedal position.         47. A method as described in paragraph 42, wherein said first         vehicle operating parameter comprises brake pressure; and the         method comprises analysing the brake pressure to generate a         third confidence value of the first estimation.         48. A method as described in paragraph 42, wherein said first         vehicle operating parameter comprises at least one wheel slip         measurement; and the method comprises analysing the at least one         wheel slip measurement to generate a fourth confidence value of         the first estimation.         49. A method as described in paragraph 48 comprising comparing         the at least one wheel slip measurement to a look-up table to         generate the fourth confidence value.         50. A method as described in paragraph 48, wherein the analysis         of the wheel slip measurement comprises comparing first and         second wheel slip measurements to a look-up table.         51. A method as described in paragraph 42 comprising determining         the filter coefficient in dependence on the generated confidence         value or on one of the generated confidence values.         52. A method as described in paragraph 51 comprising generating         a plurality of said confidence values, each confidence value         being generated in dependence on a different first operating         parameter; the method comprising generating the filter         coefficient in dependence on the generated confidence value         indicating the lowest confidence in the first estimation.         53. A method as described in paragraph 51 comprising inverting         the generated confidence value, the filter coefficient being         generated in dependence on the inverted confidence value.         54. A dynamic filtering method comprising:     -   generating a first signal and a second signal;     -   calculating a cut-off frequency;     -   applying the calculated cut-off frequency to a low-pass signal         filter and filtering the first signal using the low-pass signal         filter;     -   applying the calculated cut-off frequency to a high-pass signal         filter and filtering the second signal using the high-pass         signal filter; and     -   combining the filtered outputs of said low-pass signal filter         and said high-pass signal filter.         55. A dynamic filtering method as described in paragraph 54         comprising generating the first signal in dependence on at least         one first parameter.         56. A dynamic filtering method as claimed in claim 55, wherein         the at least one first parameter is at least one first vehicle         dynamics parameter selected from the following set: reference         velocity, longitudinal velocity, longitudinal acceleration,         lateral velocity, lateral acceleration, vertical velocity,         vertical acceleration, roll, yaw, pitch and wheel slip.         57. A dynamic filtering method as described in paragraph 54         comprising generating the second signal in dependence on at         least one second parameter.         58. A dynamic filtering method as claimed in claim 57, wherein         the at least one second parameter is at least one second vehicle         dynamics parameter selected from the following set: reference         velocity, longitudinal velocity, longitudinal acceleration,         lateral velocity, lateral acceleration, vertical velocity,         vertical acceleration, roll, yaw, pitch, and wheel slip.         59. A dynamic filtering method as described in paragraph 54         comprising calculating a first confidence value of the first         signal; and calculating the cut-off frequency in dependence on         said first confidence value.         60. A dynamic filtering method as described in paragraph 54         comprising calculating the first confidence value in dependence         on a third parameter.         61. A dynamic filtering method as claimed in claim 60, wherein         the third parameter is a vehicle dynamics parameter or a vehicle         control input. 

1. An apparatus for estimation of a vehicle state, the apparatus comprising a controller configured to: determine a first estimation of the vehicle state in dependence on at least one first vehicle dynamics parameter; determine a filter coefficient in dependence on a first vehicle operating parameter; set an operating frequency of a first signal filter in dependence on the determined filter coefficient and use the first signal filter to filter the first estimation of the vehicle state to generate a first filtered estimation of the vehicle state; and output a control signal in dependence on the first filtered estimation of the vehicle state, wherein the vehicle state is a pitch angle of the vehicle measured about a transverse axis, and wherein the at least one first vehicle dynamics parameter comprises a reference velocity along a longitudinal axis of the vehicle.
 2. (canceled)
 3. The apparatus as claimed in claim 1, wherein the first signal filter is a low-pass signal filter and the operating frequency of the first signal filter is a cut-off frequency of the low-pass signal filter.
 4. The apparatus as claimed in claim 1, wherein the at least one first vehicle dynamics parameter is one or more parameters selected from the following set: longitudinal velocity, longitudinal acceleration, lateral velocity, lateral acceleration, vertical velocity, vertical acceleration, roll, yaw, pitch and wheel slip.
 5. The apparatus as claimed in claim 1, wherein the controller is further configured to: determine a second estimation of the vehicle state in dependence on at least one second vehicle dynamics parameter; and set an operating frequency of a second signal filter in dependence on the determined filter coefficient and use the second signal filter to filter the second estimation of the vehicle state to generate a second filtered estimation of the vehicle state.
 6. The apparatus as claimed in claim 5, wherein the second estimation of the vehicle state is determined by referencing the at least one second vehicle dynamics parameter to a look-up table stored in system memory.
 7. The apparatus as claimed in claim 5, wherein the at least one second vehicle dynamics parameter is one or more parameters selected from the following set: reference velocity, longitudinal velocity, longitudinal acceleration, lateral velocity, lateral acceleration, vertical velocity, vertical acceleration, roll, yaw, pitch, and wheel slip.
 8. The apparatus as claimed in claim 5, wherein the second signal filter is a high-pass signal filter, and the operating frequency of the second signal filter is a cut-off frequency of the high-pass signal filter.
 9. The apparatus as claimed in claim 5, wherein the controller is configured to combine the first and second filtered estimations of the vehicle state.
 10. The apparatus as claimed in claim 5, wherein the second estimation of the vehicle state determines a relative body pitch angle of the vehicle. 11-22. (canceled)
 23. A dynamic filtering apparatus comprising a controller configured to: generate a first signal and a second signal; calculate a cut-off frequency; apply the calculated cut-off frequency to a low-pass signal filter and filter the first signal using the low-pass signal filter; apply the calculated cut-off frequency to a high-pass signal filter and filter the second signal using the high-pass signal filter; and combine filtered outputs of the low-pass signal filter and the high-pass signal filter.
 24. The dynamic filtering apparatus as claimed in claim 23, wherein the controller is configured to generate the first signal in dependence on at least one first parameter.
 25. The dynamic filtering apparatus as claimed in claim 24, wherein the at least one first parameter is at least one first vehicle dynamics parameter selected from the following set: reference velocity, longitudinal velocity, longitudinal acceleration, lateral velocity, lateral acceleration, vertical velocity, vertical acceleration, roll, yaw, pitch and wheel slip.
 26. The dynamic filtering apparatus as claimed in claim 23, wherein the controller is configured to generate the second signal in dependence on at least one second parameter.
 27. The dynamic filtering apparatus as claimed in claim 26, wherein the at least one second parameter is at least one second vehicle dynamics parameter selected from the following set: reference velocity, longitudinal velocity, longitudinal acceleration, lateral velocity, lateral acceleration, vertical velocity, vertical acceleration, roll, yaw, pitch, and wheel slip.
 28. The dynamic filtering apparatus as claimed in claim 23, wherein the controller is further configured to calculate a first confidence value of the first signal, and wherein the cut-off frequency is calculated in dependence on the first confidence value.
 29. The dynamic filtering apparatus as claimed in claim 28, wherein the first confidence value is calculated in dependence on a third parameter.
 30. The dynamic filtering apparatus as claimed in claim 29, wherein the third parameter is a vehicle dynamics parameter or a vehicle control input.
 31. A vehicle comprising the apparatus as claimed in claim
 1. 32. A method of estimating a vehicle state, the method comprising: determining a first estimation of the vehicle state in dependence on at least one first vehicle dynamics parameter; determining a filter coefficient in dependence on a first vehicle operating parameter; setting an operating frequency of a first signal filter in dependence on the determined filter coefficient and using the first signal filter to filter the first estimation of the vehicle state to generate a first filtered estimation of the vehicle state; and outputting a control signal in dependence on the first filtered estimation of the vehicle state, wherein the vehicle state is a pitch angle of the vehicle measured about a transverse axis, and wherein the at least one first vehicle dynamics parameter comprises a reference velocity along a longitudinal axis of the vehicle. 33-53. (canceled)
 54. A dynamic filtering method, comprising: generating a first signal and a second signal; calculating a cut-off frequency; applying the calculated cut-off frequency to a low-pass signal filter and filtering the first signal using the low-pass signal filter; applying the calculated cut-off frequency to a high-pass signal filter and filtering the second signal using the high-pass signal filter; and combining filtered outputs of the low-pass signal filter and the high-pass signal filter. 55-63. (canceled) 